First mods to do on a 2004 GT.

[420nitro]

If your worried about the warranty, then put parts on that you can take off and put back to stock. I would also go back to the dealer and ask for the service manager or director. Ask him/her what they feel comfortable on the car that would not void warrenty. Most of the time it's a judgement call on they're behalf! So be cool with them and they will most likley tell you what you can and can't do with in reason. You will be SURPRISED at how much more lenient they can be towards you later!
Hopefully he won't be a dick!
Good Luck